Been there,done that, got many many t shirts, and the bottom line is that the only solution I have ever found is to give in and put a litter tray where they are actually going.

I am sure that is not the answer you wanted to hear. There is another option but it involves money, that is to get a kitten pen, food in one corner of the pen, litter tray in the other, bed in another corner, and have them in there all the time unless they are actually being supervised.

Liz